Dear Friends,
For many times i'm searching for a good Function Generator to use in place of the good design with ICL8038, MAX038 and XR2206 because them are IMPOSSIBLE to found in Brazil.
I make the Wien Bridge with a OPA228, OPA227, OPA134, NE5534. I used styroflex capacitors, and a Wave Switch.
The capacitors are 1uf, 0.1uf, 0.01uf, 0.001uf, I tested for the potenciometer a 47k wich I have good results. The waveform choose when be good and when don't oscillate.
I put the 10kohm potentiometer and the waveform was good in these 4 scales. But... It choose when oscillate or when don't oscillate. ( The waveform wasn't so good )
Does anybody have a opinion about this? I need a Function Generator!
A digital switch is a good choice to pulll off the waveswitch ( I don't know the name of this switch in English, but it is a rotative switch )?
Best Regards,
Felipe Navarro
For many times i'm searching for a good Function Generator to use in place of the good design with ICL8038, MAX038 and XR2206 because them are IMPOSSIBLE to found in Brazil.
I make the Wien Bridge with a OPA228, OPA227, OPA134, NE5534. I used styroflex capacitors, and a Wave Switch.
The capacitors are 1uf, 0.1uf, 0.01uf, 0.001uf, I tested for the potenciometer a 47k wich I have good results. The waveform choose when be good and when don't oscillate.
I put the 10kohm potentiometer and the waveform was good in these 4 scales. But... It choose when oscillate or when don't oscillate. ( The waveform wasn't so good )
Does anybody have a opinion about this? I need a Function Generator!
A digital switch is a good choice to pulll off the waveswitch ( I don't know the name of this switch in English, but it is a rotative switch )?
Best Regards,
Felipe Navarro
What are you using to control positive feedback in the oscillator? Pots don't work! Traditionally light bulbs were used because of their positive temperature coefficient. Finding the right one might be an adventure, but seems a bit easier than using a precision rectifier and jfet as I once did. (Bulb worked better.) You might want to use one bulb for the higher frequency ranges and add another for the LF ranges or select one with a longer time constant in order to reduce LF distortion.
Look at the old HP201 oscillator, not to mention the old Thorens TD-125 (MKII) electronics as examples of bulb regulated Weinbridge oscillators.
A well implemented Weinbridge will blow the doors off of any function generator in terms of thd.
Note that the chips you listed weren't that great either. I built a generator around the XR-2206 and despite great attention to detail it never worked well. (I was an eng tech at the time I designed and built it.) Incidentally I don't think any of those types are commonly available now, IIRC most of the listed types were in production over 20yrs ago and have not stood the test of time that well..
Look at the old HP201 oscillator, not to mention the old Thorens TD-125 (MKII) electronics as examples of bulb regulated Weinbridge oscillators.
A well implemented Weinbridge will blow the doors off of any function generator in terms of thd.
Note that the chips you listed weren't that great either. I built a generator around the XR-2206 and despite great attention to detail it never worked well. (I was an eng tech at the time I designed and built it.) Incidentally I don't think any of those types are commonly available now, IIRC most of the listed types were in production over 20yrs ago and have not stood the test of time that well..
What functions, amplitude and repetition rate do you require?
You can output functions from a soundcard.
You can find some free software for tones, e.g TTG, or you can make a .wav file with any waveshape you want with a sample editor or a bit of programming. You might even be able to create the values and export it from Excel.
w
Obviously the rise time is limited and you will only get a really good square wave at fairly low frequencies, but this is true to an extent with any FG.
You can output functions from a soundcard.
You can find some free software for tones, e.g TTG, or you can make a .wav file with any waveshape you want with a sample editor or a bit of programming. You might even be able to create the values and export it from Excel.
w
Obviously the rise time is limited and you will only get a really good square wave at fairly low frequencies, but this is true to an extent with any FG.
I found this circuit a while ago, maybe it helps.
http://www.diyaudio.com/forums/showthread.php?threadid=96120
http://www.diyaudio.com/forums/showthread.php?threadid=96120
There was a project of a state variable oscillator with stabilisation by diodes which was described by Ian Hickman in Electronics World. Disto lmuch less than 0.1%, simple and interesting.
About 25 years ago, Elektor made a full project with double sided PCB using an XR2206. It got rid of the spikes on the sines at high frequencies quite well. Useful for applications where stabilisation of the output voltage is more important than distorsion (about 0.5 %), I still use it today.
About 25 years ago, Elektor made a full project with double sided PCB using an XR2206. It got rid of the spikes on the sines at high frequencies quite well. Useful for applications where stabilisation of the output voltage is more important than distorsion (about 0.5 %), I still use it today.
Go to the Linear Technology site and track down AN5. On page 8 is a simple wien bridge oscillator. The trick is using small light bulbs for stabilization. It can be done with other circuits, but they are way more trouble. For more on that, see the piece Jim Williams of LT wrote, "Max Wien, Mr. Hewlett, and a Rainy Sunday Afternoon". This is in his book, "Analog Circuit Design". Possibly a local library will have it or can get it. Another interesting route is to use the Analog Devices programmable oscillator chips. You'll need to do some software, but they go from near DC to near daylight, and the waveform quality is better than the chips of old, with their triangle-to-sine converters that always left little points on the waveform. Even my favorite Wavetek 185 sweep gen suffers slightly from that.
mod_evil said:
A small night light bulb might be a bit better, but if you are getting a decent sinewave at LF then perhaps that 40W bulb is the ticket.
Use a comparator to generate a nice square wave.. Make the slicing level adjustable and you can vary the duty cycle.
The aforementioned article by Jim Williams is highly recommended. I do ATE hardware design and he is the absolute authority on extreme precision dc source A to D and D to A converters (how about 1ppm accuracy) amongst a lot other things. While I am gushing I think Bob Pease over at National is worth more than a casual read as well.
kevinkr said:
Friends,
I tested a IRF640, with a 120ohm resistor in the Gate to make the square wave from the sine.
In the low frequencies < 1khz the square wave converter is distorting the square wave.
I will put some photos about this.
But... Now it isn't working. It works when it wants. I haven't words to write about this.
The senoidal output seems okay.
Best Regards,
Felipe Navarro
Linear Tech has some great op-amp oscillator circuits. See the various application notes that Jim Williams has written. They show you how to build an AGC with off-the-shelf parts.
Probably the easiest (relatively speaking) high quality signal generator is the Bob Cordell which appeared in Audio in the early 1980's -- this was the design used for the Tektronix SG505 which is a truely great sig gen. You can follow up the Cordell oscillator with the new buffer from National Semiconductor for a device that you can test low impedance loads with.
Cordell's design is on his website.
Probably the easiest (relatively speaking) high quality signal generator is the Bob Cordell which appeared in Audio in the early 1980's -- this was the design used for the Tektronix SG505 which is a truely great sig gen. You can follow up the Cordell oscillator with the new buffer from National Semiconductor for a device that you can test low impedance loads with.
Cordell's design is on his website.
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